A processed starch food is eaten all over the world in various forms such as breads, noodles, rice cakes and confectioneries regardless of principal food or side dish. Various techniques for improving a texture of the processed starch food are designed in order to further enhance a palatability of the processed starch food. For example, softening of breads, prevention of dried out due to aging, addition of firmness (elasticity) to noodles, improvement of stickiness, improvement of chewiness, and the like are reported.
Various techniques for improving a quality of the processed starch food by adding soybean protein are designed. Patent document 1 describes an improvement of bread, sponge cake, steamed bread and rusk. Patent document 2 describes a method for improving deep fry batter. However, these effects are narrow and more improvement has been desired.
Generally, soybean protein is used as a soybean protein powder obtained by pulverizing a soybean protein raw material such as whole soybean and defatted soybean, or a water extract obtained by extracting with water or warm water and then removing fiber. The water extract is often subjected to further treatment and used as a soybean protein isolate. For example, the water extract is subjected to an isoelectric point precipitation at pH about 4 to 5, and the precipitated protein is separated and recovered, and water is added to the precipitate, and the precipitate is re-dissolved by neutralizing to pH about 7 to 8 with alkaline agent such as sodium hydroxide. And thereby, soybean protein is concentrated to obtain soybean protein isolate. In general, soybean protein has high function and is insusceptible to inhibitory effect by another component when the protein content is high. However, a lot of complex processes are necessary for the separation and refinement. In addition, it has problems in respect of the negative environmental impact and the cost of manufacturing, for example, large quantity of water is used. Therefore, separation and refinement technology having a little process as much as possible has been required.